The invention relates generally to pressure sensors and more particularly to pressure responsive variable parallel plate capacitive transducers. Such transducers are shown and described, for example, in U.S. Pat. No. 4,716,492, assigned to the assignee of the present invention. A capacitive transducer is shown in the patent having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation on a ceramic base, with metal coatings deposited on respective opposing surfaces of the diaphragm and the base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electrical circuit connected to the transducer terminals is mounted on the transducer. A cup-shaped connector body of electrical insulating material is fitted over the electrical circuit and is secured to the transducer by a housing sleeve which has a port for exposing the transducer diaphragm to an applied pressure. The diaphragm is movable in response to variations in pressure applied to the diaphragm to vary the capacitance of the capacitor and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
In order to maximize the economies of mass production to lower the transducer cost and thereby make such transducers economically feasible for a wide number of applications, including many previously served by low cost mechanical transducers, a standard size package is selected small enough to be received in a large number of applications yet large enough to provide a reliable signal. The size of the package determines the maximum size of the capacitor plates which, along with the gap between the plates, determines the capacitance signal. This results in limiting the size of the capacitor plates to a smaller size than would be ideal for many applications and relying on the electrical circuit to properly condition the signal. The circuit, on the other hand, requires a minimum level of capacitance for it to be able to effectively condition the output signal and this in turn affects the distance or gap required between the capacitor plates to produce the minimum capacitance level. In transducers of the type disclosed in U.S. Pat. No. 4,716,492 distances between the plates are in the order of 10-17 microns.
One approach described in the above patent to provide this selected gap employs a cup-shaped member having a relatively rigid rim secured to a base substrate placing a bottom of the cup in selected, spaced, overlying relation to a capacitor plate on the base substrate. An electrically conductive layer is disposed on the inner surface of the cup bottom to provide the second capacitor plate with the bottom being resiliently flexible to serve as a diaphragm to move the second plate toward and away from the first capacitor plate in response to variations in fluid pressure applied to the outer surface of the bottom of the cup. The configuration of the cup-shaped member, however, is not conducive to low cost manufacturing techniques. Due, in part, to the small sizes involved it is very difficult to obtain consistent flat surfaces on the cup bottoms which are parallel to the substrate surface. Slight variations from device to device cause changes in capacitance signals produced by the transducers which frequently fall outside the window of values acceptable by the signal conditioning circuitry.
Another approach described in the above patent employs a flat diaphragm element secured to the base substrate in selectively spaced relation thereto by disposing a spacing and securing medium such as a mixture of glass frit including a plurality of balls of glass of selected diameter between the flat diaphragm and the substrate at the periphery of the diaphragm. The glass frit is selected to be fusible at a first temperature at which the balls remain unfused and the mixture is then heated to the fusing temperature of the frit to secure the diaphragm to the substrate at a spacing from the substrate determined by the diameter of the balls. The provision of flat surfaces which extend over the entire diaphragm as well as the base substrate is very conducive to consistent, reproducible results from device to device; however, the flat surfaces generally require grinding to ensure that the surfaces are parallel to one another. Further, the use of the glass material to both space and secure the diaphragm to the base results in undesirable yield losses due to various factors such as unevenness sometimes occurring due to imperfections in the grinding process, variations in the compressive force used to clamp the diaphragms to the base when the device is fired to fuse the glass and other process variables such as the specific temperature profile of the firing and the specific glass composition employed.
In U.S. Pat. No. 5,044,202, assigned to the assignee of the instant invention, a curved recess is formed in the base over which a flat, flexible diaphragm is disposed. The spacing between capacitor plates deposited on the diaphragm and a central portion of the recess is determined by the curvature of the recess. Sealant material such as glass is disposed on the outer marginal portion of the curved surface of the recess. While this structure provides a reliable, accurate sensor it requires an extra grinding operation to form the recess which adds to the expense of the device.
In copending U.S. patent application Ser. No. 07/972,680, assigned to the assignee of the instant invention, a pressure responsive, variable parallel plate capacitive transducer is shown and described comprising a body of ceramic material having a cavity formed therein closely adjacent an outer surface thereof. Metal capacitor plates are deposited on opposite sides of two surfaces defining the cavity with vias extending to terminal areas. The ceramic comprises conventional material such as 80% by weight alumina up to essentially 100% with the balance being additives to form a glass at a sintering temperature. The ceramic is provided in powdered form coated with an organic binder, as a spray dried powder, ready for pressing into any selected configuration. First and second portions, i.e., a diaphragm and a base having a recess formed in an outer face surface, are formed by pressing the powder in a die. In one embodiment, metallized coatings of high temperature material such as tungsten in the form of a thick film paste, are deposited as by screen printing on one surface of the diaphragm portion and on the recessed outer face surface of the base portion. The vehicle used in the thick paste is then removed, as by heating. Spacer means of organic material may be placed in the recess to ensure that the cavity gap is maintained during the following pressing step. The two portions are then pressed together to form a single unit and then the unit is heated in an air atmosphere to a first debinderizing temperature. After the organics, including the spacer means, are vaporized/decomposed and released through the still open cells of the ceramic, the unit is placed in a high temperature oven and co-fired in a reducing atmosphere with the metal layers forming a conductive coating bonded to the ceramic and the ceramic being sintered together to form a monolithic, closed cell body.
According to a modified embodiment, low temperature ceramic materials can be used for the ceramic which can be sintered at a temperature low enough to permit the use of conventional printed circuit inks fired in an air atmosphere.
Such monolithic transducers offer many advantages regarding cost, reliability and improved yield in manufacturing. When used with a signal conditioning circuit of the type referenced above with regard to U.S. Pat. No. 4,716,492, a reference capacitor is also employed as well as means for providing temperature compensation in order to achieve desired accuracy over a selected temperature range.